This invention relates to a catalytic process for hydroconversion of heavy hydrocarbon streams containing asphaltenic material, metals, sulfur-containing compounds and nitrogen-containing compounds. More particularly, this invention relates to a hydroconversion process using a catalyst mixture having improved activity and activity maintenance in the desulfurization, demetallation and denitrogenation of heavy hydrocarbon streams, which produce insoluble carbonaceous substances also known as Shell hot filtration solids, dry sludge, and hexane insolubles.
While crude oil prices have declined since the sharp rises in 1973 and 1979 that spurred efforts to improve heavy hydrocarbon conversion, in the long-term it may become necessary to increasingly utilize heavy crudes due to a decreasing supply or availability of light oils. Thus, as refiners increase the proportion of heavier, poorer quality crude oil in the feedstock to be processed, the need grows for effective processes to treat the fractions containing increasingly higher levels of metals, asphaltenes, sulfur, and nitrogen.
It is widely known that various organo-metallic compounds and asphaltenes are present in petroleum crude oils and other heavy petroleum hydrocarbon streams, such as petroleum hydrocarbon residua and hydrocarbon streams derived from tar sands. The most common metals found in such hydrocarbon streams are nickel, vanadium, and iron. Such metals are very harmful to various petroleum refining operations, such as hydrocracking, hydrodesulfurization, and catalytic cracking. The metals and asphaltenes cause interstitial plugging of the catalyst bed and reduced catalyst life. The various metal deposits on a catalyst tend to poison or deactivate the catalyst. Moreover, the asphaltenes tend to reduce the susceptibility of the hydrocarbons to desulfurization. If a catalyst, such as a desulfurization catalyst or a fluidized cracking catalyst, is exposed to a hydrocarbon fraction that contains metals and asphaltenes, the catalyst will become deactivated rapidly and will be subject to premature replacement.
Although processes for the hydrotreating of heavy hydrocarbon streams, including but not limited to heavy crudes, reduced crudes, and petroleum hydrocarbon residua, are known, the use of fixed-bed catalytic processes to convert such feedstocks mostly to lighter products without appreciable asphaltene precipitation and reactor plugging and with effective removal of metals and other contaminants, such as sulfur compounds and nitrogen compounds, are not common because the catalysts employed have not generally been capable of maintaining activity and performance.
Thus, the subject hydrotreating processes are most effectively carried out in an ebullated bed system where a portion of the deactivated catalyst is replaced with fresh catalyst at a desired rate. In an ebullated bed, preheated hydrogen and resid enter the bottom of a reactor wherein the upward flow of resid plus internal liquid recycle suspend the catalyst particles in the liquid phase. Recent developments involved the use of a powdered catalyst which can be suspended without the need for a liquid recycle. In this system, part of the catalyst is continuously or intermittently removed in a series of cyclones and fresh catalyst is added to maintain activity. Roughly about 1 wt. % of the catalyst inventory is replaced each day in an ebullated bed system. Thus, the overall system activity is the weighted average activity of catalyst varying from fresh to very old, i.e., deactivated.
The art discloses a multitude of catalyst systems suitable for upgrading heavy hydrocarbons. A majority of these processes are two-stage processes wherein the first-stage catalyst is effective in metals removal, asphaltene reduction and hydrocracking, whereas the second-stage catalyst is effective in hydrogenation reactions such as desulfurization, denitrogenation, reduction of Conradson carbon and saturation of aromatics. This same concept is employed in single fixed bed reactors where layers or zones of the various stage catalysts are situated in series. These two-stage catalyst processes are typically carried out by varying the catalytic metals, catalytic metals loadings, pore size distributions and support compositions of the catalyst employed in each stage, zone, or layer.
For instance, a paper entitled "Stacked Bed Hydrotreating Catalyst Technology" presented at the 1986 NPRA Annual Meeting Mar. 23-25, 1986 (Charles T. Adams, Don M. Washecheck, Richard H. Stade, W. J. Daniels), discloses a first stage catalyst that contains nickel and molybdenum to enhance aromatics hydrogenation and a second stage catalyst that contains cobalt and molybdenum to enhance hydrodenitrogenation, thus employing a system where catalytic metals are varied.
There are numerous examples of the approach wherein pore size distributions are varied in the catalyst stages, especially where the upstream stage catalyst utilizes relatively larger pore sizes.
U.S. Pat. No. 4,830,736 (Hung et al.) discloses a "graded" system wherein a hydrodemetallation catalyst is composed of different types of catalysts with differing metals capacities and hydrogenation activities to provide gradual change through the catalyst system in the direction of the flow. Specifically, a process is disclosed wherein a first zone comprises catalyst particles having at least 10 volume percent of their pore volume above 1,000 Angstroms and a surface area ranging from about 50 m.sup.2 /g to about 200 m.sup.2 /g followed by a second zone that comprises catalyst particles having less than 20 volume percent of their pore volume in pores having a diameter of 1,000 Angstroms in diameter, an average mesopore diameter ranging from about 80 Angstroms to about 400 Angstroms and a surface area ranging from about 80 m.sup.2 /g to about 300 m.sup.2 /g.
U.S. Pat. No. 4,297,242 (Hensley et al.) discloses a multiple-stage catalytic process for hydrodemetallization and hydrodesulfurization of heavy hydrocarbon streams containing asphaltenes and a substantial amount of metals wherein the pore sizes of the catalysts in the respective stages are varied. The first stage of this process comprises contacting the feedstock in a first reaction zone with hydrogen and a demetallation catalyst comprising hydrogenation metal selected from Group VIB and/or Group VIII deposed on a large-pore, high surface area inorganic oxide support; the second stage of the process comprises contacting the effluent from the first reaction zone with a catalyst consisting essentially of hydrogenation metal selected from Group VIB deposed on a smaller pore, catalytically active support comprising alumina, said second stage catalyst having a surface area within the range of about 150 m.sup.2 /gm to about 300 m.sup.2 /gm, an average pore diameter within the range of about 90 Angstroms to about 160 Angstroms, and the catalyst has a pore volume within the range of about 0.4 cc/gm to about 0.9 cc/gm.
In U.S. Pat. No. 4,212,729 (Hensley et al.), another two-stage catalytic process for hydrodemetallization and hydrodesulfurization of heavy hydrocarbon streams containing asphaltenes and metals is disclosed. In this process, the first-stage demetallation catalyst comprises a metal selected from Group VIB and from Group VIII deposed on a large-pore, high surface area inorganic oxide support. The second stage catalyst contains a hydrogenation metal selected from Group VIB deposed on a smaller pore catalytically active support having the majority of its pore volume in pore diameters within the range of about 80 Angstroms to about 130 Angstroms.
U.S. Pat. No. 4,626,340 (Galiasso et al.) discloses a process for the conversion of heavy hydrocarbon feedstocks wherein the feedstock is passed to a hydrodemetallization zone having a bimodal catalyst, followed by a thermal cracking zone, and then followed by a hydrocarbon conversion zone containing a bimodal catalyst. The hydrodemetallization zone bimodal catalyst possesses a pore-distribution such that 20% of the pores are between 10 and 100 Angstroms and 60% of the pores are between 100 and 1,000 Angstroms. The hydrocarbon conversion zone bimodal catalyst possesses a pore size distribution such that 40% of the pores are between 10 and 100 Angstroms and 40% are between 100 and 1,000 Angstroms.
U.S. Pat. No. 4,016,067 (Fischer et al.) discloses another two-stage process for upgrading residual oil fractions. The first stage contains a catalyst having at least 60% of its pore volume in pores of 100 Angstroms and 200 Angstroms and at least about 5% of its pore volume in pores having a diameter greater than 500 Angstroms. The second-stage catalyst has a major fraction of its pores in the 30 to 100 Angstroms diameter range.
Similarly, U.S. Pat. No. 4,447,314 (Banta) also discloses a two-stage process for hydrotreating residual oil fractions. Specifically, the first-stage catalyst has at least 60% of its pore volume in pores with diameters of about 100 to 200 Angstroms, at least 5% of its pore volume in pores greater than 500 Angstroms, and a surface area up to about 110 m.sup.2 /g. The second-stage catalyst has a surface area of at least 150 m.sup.2 /g and 50% of its pore volume in pores with diameters of 30 to 100 Angstroms.
A contrary approach is disclosed in U.S. Pat. No. 4,789,462 (Byrnes et al.), where a "reverse-graded catalyst system" is disclosed. In particular, the process catalyst system comprises two or more catalyst layers in which at least two successive catalyst layers are characterized as having decreasing desulfurization activity and increasing average macropore diameter in the direction of hydrocarbon flow.
Yet another approach used by refiners to upgrade heavy hydrocarbons has been to use catalysts having bimodal pore size distributions in the entire reactor. A bimodal distribution means a pore distribution including two major peaks of pore diameters measured as a plot of pore volume in cc/g versus pore diameter or radius. Typically, the macropore peak occurs where the pores have diameters in excess of 1,000 Angstroms, whereas the smaller pore or mesopore peak occurs at pore sizes ranging from 100 to 200 Angstroms.
For instance, U.S. Pat. No. 4,707,466 (Beaton et al.) discloses a bimodal catalyst effective for the hydrodemetallization, hydrodesulfurization and hydrocracking of a hydrocarbon feedstock containing asphaltenes, metals, and Shell hot filtration solids precursors.
The prior art also discloses catalyst schemes wherein physical or mechanical mixtures of sets of catalysts having different catalytic metals loadings or pore size distributions are employed.
U.S. Pat. No. 4,353,791 (Pellet) discloses a coal liquefaction hydrotreating catalyst composition comprising particles of component A consisting essentially of at least one Group VIB metal component and component B consisting essentially of either a cobalt and/or a nickel component. Both component A particles and component B particles are supported on the same high surface area porous refractory inorganic oxide having a bimodal pore size distribution. The subject reference defines the smaller pores as ranging from 100 to 200 Angstroms in diameter with the larger pores generally ranging from 1,000 to 10,000 Angstroms.
Offenlegungsschrift, D. E. 3207554 Al discloses a process for desulfurization and demetallization of oils using a mixture of two catalyst components having slightly differing pore size distributions. In particular, the subject reference discloses a first catalyst having an average pore diameter ranging from about 45 to about 75 Angstroms and a second catalyst having an average pore diameter ranging from about 85 to about 125 Angstroms wherein both the second and first catalyst have less than 5% of the total pore volume in pores having diameters greater than 500 Angstroms. Neither of the catalysts employed in the mixture include any appreciable macropore volume where macropores are generally defined as pores having a diameter greater than 1,000 Angstroms.
The catalysts disclosed in the above references generally contain hydrogenating components comprising one or more metals from Group VIB and/or Group VIII on a high surface area support such as alumina, and such combinations of metals as cobalt and molybdenum, nickel and molybdenum, nickel and tungsten, and cobalt, nickel, and molybdenum have been found useful. Generally, cobalt and molybdenum have been preferred metals in the catalysts disclosed for hydrotreatment of heavy hydrocarbon streams, both in first-stage catalytic treatment to primarily remove the bulk of the metal contaminants, and in second-stage catalytic treatment primarily for desulfurization.
The treatment of the heavy hydrocarbon streams, such as resids, described above, presents a myriad of difficulties for the refiner. Specifically, catalysts having the highest activity for denitrogenation and desulfurization also tend to deactivate rapidly. The rapid deactivation occurs because the high-activity catalysts typically have pores having relatively small average pore diameters, wherein the entrances or mouths to these pores are quickly blocked by the relatively large species present in heavy hydrocarbon streams, such as asphaltenes, organo-metallics, etc. For instance, catalysts having substantial surface area in pores having diameters less than about 200 Angstroms restrict organo-metallics and asphaltenes from the active sites in the pores because these species become diffusionally hindered, whereas smaller less refractory components diffuse unhindered.
On the other hand, catalysts having relatively large average pore diameters provide superior demetallization, asphaltene conversion and Shell hot filtration solids precursors conversion. For instance, catalysts having surface area in pores larger than 200 Angstroms permit deposition of metals and asphaltenes within such pores by virtue of the shorter diffusion path. Increases in pore diameter however mean a lower surface area which engenders a loss in catalytic activity.
In order to upgrade heavy hydrocarbons, the trade-offs set out above need to be resolved in a satisfactory manner. Thus as mentioned above, this dilemma has typically been addressed by using bimodal catalysts where the macropores serve as diffusing feeder channels for the large molecules or varying catalysts in series in the direction of hydrocarbon flow.
Another difficulty which arises in resid or heavy oil hydroprocessing units is the formation of insoluble carbonaceous substances also known as Shell hot filtration solids. These substances cause operability problems in the hydrotreating and downstream units. Certain resids tend to produce greater amounts of solids thereby limiting the level of upgrading by the amount of these solids the hydroprocessing unit can tolerate.
Further, the higher the conversion level for given feedstocks the greater the amount of solids formed. In high concentrations, these solids accumulate in lines and separators, causing fouling, and in some cases interruption or loss of process flow. The formation of these solids results in the agglomeration of the catalyst, thereby causing high pressure drops through fixed catalyst beds. In an ebullated bed type reactor, catalyst agglomeration can prevent proper mixing of the oil, hydrogen, and catalyst which allows uncontrolled reactions and local hot spots that can result in reactor failure, serious fires, or explosion.
To avoid these problems, refiners have taken several measures. Conversion has been limited to 40 to 70 volume % or solids have been removed after a partial initial conversion of the feedstock prior to further conversion. Further, refiners have been limited in their choice of feedstocks by having to avoid the use of or limit the conversion of feedstocks that have a greater tendency to produce the subject solids.
Accordingly, it is a general object of the present invention to provide a process affording superior demetallization, desulfurization, hydrodenitrogenation, and hydroconversion of a hydrocarbon feedstock containing metals, sulfur, nitrogen, and Shell hot filtration solids precursors.
It is another general object of this invention to provide a process affording a higher conversion level for heavy hydrocarbon feedstocks that tend to form greater amounts of insoluble substances, especially that fraction of the feedstock that boils over 1,000.degree. F.
Yet another object of the present invention is to provide a process that produces gas oil effluent streams that can be catalytically cracked to gasoline fractions at higher yield levels.
Another object of the present invention is to provide a process that produces effluent distillates having lower nitrogen and sulfur contents affording such distillates higher value in subsequent blending processes.
Another object of the present invention is to provide a process that provides greater liquid volume expansion of effluent fractions such as naphtha and distillate.
Surprisingly, it has been discovered that the above objects can be obtained when a physical or mechanical mixture of catalysts is used in accordance with the present invention wherein a first set of catalyst particles contains a prescribed maximum average pore diameter and a second set of catalyst particles contains a prescribed amount of macropore volume.
Specifically, by using a catalyst mixture in accordance with the present invention hydrocarbon streams containing metals, sulfur, nitrogen, asphaltenes and Shell hot filtration solid precursors can be upgraded with greatly reduced operability difficulties. By using a first relatively small-pore highly active catalyst mixed with a second catalyst having a large volume of macropore volume, the metals and asphaltenes preferentially deposit in the large pore catalyst, thereby permitting the more active small pore catalyst to upgrade smaller feed components without being deactivated by pore entrance blocking.